8.19\ Auditory Brainstem

Stimulator

8.19.1\ Discussion

Auditory brainstem implants (ABIs) are used to provide some form for hearing capacity when the contralateral ear provides no hearing or if there is concern of contralateral hearing loss, such as in neurofibromatosis Type 2 patients. The components of the ABI are analogous to cochlear implants and include a receiver-stimulator and electrode array. The electrodes are implanted via

the lateral recess of the fourth ventricle adjacent to the lateral aspect of the cochlear nucleus via a translabyrinthine or retrosigmoid approach (Fig. 8.90).

Complications related to ABI insertion include suboptimal production of auditory stimuli, cerebrospinal fluid leak along the course of the wire, and nonauditory stimuli, such as trigeminal neuralgia. Thin-section CT may be used to evaluate ABIs after implantation, although precise localization can be limited by metallic streak artifacts. Newer ABI models do not contain magnetic components and are MRI compatible.

Fig. 8.90  Auditory brainstem stimulator. The patient has a history of neurofibromatosis Type 2 and left-sided hearing loss. Scout image (a) shows the receiver-stimulator and electrode tip (arrow) in the posterior fossa. Axial CT image (b) shows the auditory brain stimulator electrode

(arrow) positioned in the left cerebellopontine angle. T2-weighted spin echo (c) and GRE (d) MRI sequences also show the tip of the electrode (arrows) in the left cerebellopontine angle, which is more conspicuous on GRE due to blooming effects

8.20\ Repair of Perilymphatic

Fistula

8.20.1\ Discussion

Symptomatic perilymphatic fistulas can be treated via surgical repair. Closure can be obtained using packing materials such as temporalis fascia, which appears as soft tissue attenuation on CT (Fig. 8.91). The main complication is

a

fistula recurrence, which occurs in 8–47% of cases. It is important to note that there may not be an imaging correlate for recurrent perilymphatic fistulas, although graft displacement can sometimes be observed. Temporal bone CT may be useful to evaluate recurrent symptoms following repair, whereby the presence of middle ear opacification beyond the round window niche may indicate recurrent fistula.

b

Fig. 8.91  Repair of perilymphatic fistula. The patient has a history of round window perilymphatic fistula repair, status post transcanal exploration and closure. Axial (a)

and coronal (b) CT images demonstrate temporalis fascia packing in the round window niche (arrows)

8.21\ Endolymphatic Sac

Decompression

and Shunting

8.21.1\ Discussion

Endolymphatic sac decompression and shunting have been used for treating intractable vertigo in patients with Meniere’s disease. The procedure consists of performing mastoidectomy and exposing the plate of the bone overlying the sigmoid sinus and posterior cranial fossa dura. These changes are readily depicted on temporal

bone CT (Fig. 8.92). The surgery can be limited to decompression alone, in which the endolymphatic sac is not opened. Alternatively, the endolymphatic sac can be incised and drained into either the mastoid or subarachnoid space, whereby a communication is created between the sac and the basal cistern. Silicone shunt tubing or valves can also be inserted and are sometimes visible on temporal bone CT. It is also common to see the bone over the adjacent posterior semicircular canal to be intentionally thinned, but hopefully not violated, as a consequence of this surgery.